Flight tests so far have produced a few in-flight problems, including one particular event that "could have been very bad", Fischer said.

In that incident, an F-22 suffered a brief, dual-engine flameout while performing a negative-g, 360 degree roll with eight SDBs loaded in the weapons bay. The flameout occurred because the aircraft entered the manoeuvre with an incorrect trim setting.

Although the engines restarted before the pilot was even aware of the problem, the momentary power loss dropped the telemetry signals to the control room on the ground.

The control staff lost contact "with the aircraft pointed down toward the water at a very low altitude", said Capt Harris Hall, a programme official. "For a time, we thought the aircraft was lost."

However, the engines were restarted almost immediately and the pilot remained in control of the aircraft, he said. The pilot landed at Edwards AFB, California, without incident. The dual-engine flameout was not discovered until flight engineers reviewed performance data from the flight.